Power transistors such as Metal Oxide Semiconductor Field Effect transistors (MOSFET) and Insulated Gate Bipolar Power transistors (IGBT) are used to switch high currents in devices such as internal combustion engine spark-ignition controllers, electric motor controllers, power inverters, and DC-DC converters. These types of power transistors are characterized as having control input threshold voltage (e.g. gate threshold voltage) that varies with temperature, and vary from part to part due to manufacturing variations. Knowing the threshold voltage levels for each of one or more individual power transistor in these devices, and being able to track changes in the threshold voltages as their operating temperatures vary provides a means for more optimal control of the power transistors switching characteristics such as voltage or current slew rates, off-to-on and on-to-off transitions times, or predicting the instant when the power transistor current starts and stops. Optimizing the control of power transistors has become particularly important as power transistor power handling and switching frequency capability continue to increase. Examples of how a threshold voltage is used to operate a power transistor are given in U.S. Pat. No. 7,675,346 issued Mar. 9, 2010 to Kesler, the entire contents of which are hereby incorporated by reference herein.